how much does a tex2D/texCUBE cost?

any concept of that?

 

It really depends on what kind of GPU you're targeting, and if the texture coordinates change in the shader, based on input data.

For Example, page 36 of these slides might be interesting to you, if you're developing for the ImgTec SGX GPU.

 

On IOS(PowerVR), "non dependent " texture samples is nearly free, dependent ones cost a lot more,

what does "dependent" texture samples mean ?
like this: tex2D(_MainTex,uv+float2(0.1,0.1)) ?

 

Yes, this is a dependent one. Also tex2D(_MainTex, uv.zw)) is a dependent one.

A dependent texture read is a texture read in which the texture coordinates depend on some calculation within the (fragment) shader instead of on a varying. As the values of this calculation cannot be known ahead of time it is not possible to pre-fetch texture data and so stalls in shader processing occur.

If your shader requires more than one dependent texture read:

When a dependent texture read occurs the current thread running on the USSE within the SGX is suspended until the comparatively slow read is completed, and another thread is swapped in. By re-arranging the code within the shader so that the two dependent texture reads were back-to-back, another performance improvement was gained. This works because the compiler can now batch these dependent texture reads together, whereas previously the shader looked as follows:

• Mathematics
• Texture Read
• Wait
• Mathematics
• Texture Read
• Wait

It now looks as follows:

• Mathematics
• Texture Read
• Texture Read
• Wait

This effectively hides some of the wait time of the first texture read behind the execution time of the second texture read. Unfortunately a wait of some description is inevitable due to the limitations of dependent texture reads. Nonetheless, the strong parallelism of the PowerVR SGX hardware ensures that these waits can be used effectively by other threads


^ But don't forget to check the code in the compiled shader and/or use #pragma glsl_no_optimize when you use more than one dependent texture read.

 

Thanks, all you, guys